Spectrum effectiveness is a net bit rate, or a ratio of a maximum throughput to a bandwidth of a communication channel or a data link, and is a key indicator of communications system design. Higher spectrum effectiveness leads to a higher communication rate and a higher user capacity, and the like, thereby improving performance of an entire communications system.
Specifically, spectrum effectiveness may be usually improved by compressing a spectral width at present. However, in this manner, intersymbol interference (ISI) is caused, degrading system performance. In view of this problem, a digital signal processing technology may currently be used on a transmitter side, and a finite impulse response filter is used to compress a signal bandwidth through pre-shaping. Different from a structure used by a conventional communications system: an equalizer+a phase recovery module+a forward error correction (FEC) module, for this ultra-narrowband signal, after the phase recovery module and before the FEC, a post filter and a sequence detector are usually added to resist the ISI and improve the system performance, specifically, as shown in FIG. 1.
Specifically, the sequence detector may perform sequence detection on a signal output by the post filter, so as to resist the intersymbol interference caused when the post filter performs noise filtering on the signal, so that sequence symbols in the signal are correctly recovered. The sequence detector outputs corresponding sequence detection soft value information to the FEC, and the FEC performs error detection on the sequence detection soft value information to recover an original signal. Further, soft value information may be exchanged in a turbo iteration manner between the sequence detector and the FEC, so as to improve the system performance. For example, the FEC may feed back corresponding error detection soft value information to the sequence detector, so that the sequence detector may further perform, based on the error detection soft value information fed back by the FEC, sequence detection on the signal output by the post filter and output new sequence detection soft value information to the FEC, and the FEC performs error detection on the new sequence detection soft value information based on the previous error detection soft value information, so as to more accurately recover the original signal. The soft value information may be information about a probability that the sequence symbols in the signal are a specified symbol (such as 0 or 1), and/or information about a ratio of probabilities that the sequence symbols are different specified symbols (for example, a ratio of a probability for 0 to a probability for 1), and so on.
However, for a bandwidth-limited system (such as a system in which the ISI exists), a received signal generally may be greatly degraded. This may be specifically reflected as a decrease in output signal quality of the equalizer. When phase recovery is performed on such a signal by using the phase recovery module, cycle skipping is frequently caused, and an incorrect signal generated is transmitted to the post filter. Consequently, input signal quality of the post filter is degraded. In addition, it may be learned from FIG. 1 that input of turbo iteration is from output of the post filter. If the input signal quality of the post filter is degraded, iteration between the sequence detection and the FEC cannot improve or cannot greatly improve the input signal quality of the post filter, resulting in a limitation on a compensation capability of the turbo iteration. Therefore, performance improvement of the entire system is limited.
In other words, when the existing turbo iteration manner is used to improve the system performance, there exists a problem that performance improvement of the entire system is limited because the input signal quality of the post filter cannot be improved or cannot be greatly improved. As a result, a system performance improvement effect is not good.